An ultra high-speed optical pulse signal can be generated by time-division-multiplexing a plurality of optical pulse signals (tributary channels), each carrying a data at the same standard bit rate (a base rate). For instance, when the base rate is 10 Gbps and the multiplexed number is 16, an ultra high-speed optical pulse signal at 160 Gbps can be generated. In a case that an ultra high-speed optical pulse signal having a single wavelength is desired, a plurality of low-speed optical pulse signals at the same base rate should be generated by dividing an output light from a single laser light source.
It is impossible to directly convert such an ultra high-speed optical pulse into an electric signal at a receiving terminal. Accordingly, it is necessary to demultiplex an optical pulse signal of each tributary channel out of an optical pulse signal input from an optical fiber transmission line. For an apparatus to demultiplex a low-speed optical pulse signal from an ultra high-speed optical pulse signal at 160 Gbps or more, a special ultra high-speed optically controlled optical switch such as an optically controlled optical switch using NOL (Nonlinear Optical Loop Mirror) and an optically controlled optical switch using SMZI (Symmetric Mach-Zehnder Interferometer) has been proposed.
The reference: E. Hilliger, et al., “EAM with Improved Switching Performance by Self Cascading,” OFC 2003, Tuesday afternoon, Vol. 1, pp. 268-269 discloses an optical pulse demultiplexing switch in which an optical loop is disposed on one end of an EAM (electroabsorption modulator) so that a signal light reciprocates in the EAM. Because the extinction factor improves as a signal light reciprocates in the EAM, a switching window can be narrowed down.
In conventional optically controlled optical switches, pulse demultiplexing characteristics are greatly affected by fluctuations of polarization and phase of both an ultra high-speed optical pulse signal and a control optical pulse signal. Furthermore, since these conventional optically controlled optical switches use nonlinear interfering effects, the adjustment of polarization and the adjustment of phase are depending on each other and therefore there are a large number of pseudo optimum points. Accordingly, it is very difficult to adjust the polarization and phase into the optimum condition.
In a configuration in the above reference, a signal light sometimes oscillates on an optical feedback due to the crosstalk between ports of an optical circulator disposed on one end of an EAM to make the adjustments of polarization and phase difficult.